The presence of hydrocarbon gas in air is conventionally monitored by beaming infra-red radiation through a chamber containing the gas-air mixture and detecting for absorption, or the degree of absorption, of radiation within the chamber. An output signal is derived which provides a measure of the amount of infra-red radiation that impinges on a detector, with an increase in the level of the output signal signifying a reduction in the amount of infra-red absorbing gas within the chamber. A bandpass filter is normally located between the radiation source and the detector to permit selective detection of the presence of a specific gas, that is a gas which absorbs radiation having a frequency in the filter pass band.
Determination of the volumetric proportion of hydrocarbon gas in air is determined by comparing a measured level of the output signal with a reference level, whilst accommodating variables introduced by ambient temperature change, component heat contributions, drifting of source and detector sensitivity, and accumulation of pollutants on optical elements within the system. One of the steps taken toward accommodating variables has involved driving the infra-red source by a square wave voltage, typically at a rate in the range of 0.5 to 50 Hz, and thermal drifts have been accommodated by measuring the peak-to-peak value of the output signal.